1. Field of the Invention
This invention relates to polycrystalline ceramic bodies of Si.sub.3 N.sub.4, and more particularly relates to such bodies having a substantially crystalline intergranular phase, and also relates to methods for preparing such bodies.
2. Prior Art
Si.sub.3 N.sub.4 powder characterized by cation impurities of 0.1 weight percent or less, a morphology of predominantly crystalline alpha phase and/or amorphous phase and fine particle size (3 microns or less average particle size as determined by B. E. T.), when consolidated with an additive such as MgO or Y.sub.2 O.sub.3 and sintered, is known to enable production of polycrystalline bodies approaching theoretical density. See U.S. Pat. No. 4,073,845, issued to S. T. Buljan, et al. on Feb. 14, 1978, and assigned to GTE Sylvania Incorporated. Such powders may be consolidated into dense bodies by either hot pressing at less severe temperature and pressure conditions than are necessary with some less pure and less reactive powders (see U.S. Pat. No. 3,830,652 issued to G. E. Gazza) or by cold pressing and sintering, which is not possible with some less pure and less reactive powders. In the fabrication of such polycrystalline bodies, up to 25 weight percent of yttrium oxide or a lanthanide rare earth oxide such as CeO.sub.2 is typically added as a sintering or densifying aid. While such modifying additives are thought to deleteriously affect maximum attainable high temperature strength and creep resistance, nevertheless such additives are thought essential to achieve highest densities, that is, densities approaching theoretical density, which are also critical to the achievement of high temperature strength.
Unfortunately, it has been found that certain dense Si.sub.3 N.sub.4 bodies containing densifying additives, while exhibiting excellent mechanical strength at room temperature and elevated temperatures up to about 1,000.degree. C., undergo significant and sometimes drastic reductions in strength at temperatures above 1,000.degree. C. and particularly above 1200.degree. C.